1. Field of the Invention
This invention relates to the control of temperature in molding apparatus, and more particularly to a molding apparatus or thermoforming plastic sheet material having improved temperature control throughout the apparatus.
2. Description of the Prior Art
Heretofore, it has not been practical to employ plastic molds for thermoforming plastic sheeting. Plastic and synthetic resin dies have been considered in connection with the molding of small articles, but such dies have not been adapted to the molding of larger articles for various reasons including, particularly, adequate temperature control. Synthetic resin materials such as epoxy or phenolic resins which might possibly be used in the dies are heat and electrical insulators of a type which increases the temperature control problem. The poor heat conductivity of such material would ordinarily materially increase the molding cycle in view of the additional time required to heat or cool the entire body of the resin. Although it has been suggested that using an epoxy resin filled with metal particles would impart semi-thermal conductivity to the mold, it has been found that conventional potting resins are too poor in heat transfer characteristics even when a metal powder or alumina filler is used. It has further been suggested that mounting or embedding cooling coils in the body of the resin through which water is circulated will enhance the temperature control characteristics of the mold. It has been found, however, that even when the body of the resin is cooled by such coils embedded therein and is filled with metal particles the heat transfer problem is not alleviated to the extent necessary to permit high speed operation of the mold. The practicability of any commercial molding apparatus is greatly dependant upon the speed and efficiency with which such apparatus can be operated.
U.S. Pat. Nos. 2,869,179 and 2,907,070 disclose molding apparatus of the type described above in which the body of the resin which forms the mold is filled with metal particles and has cooling coils through which water is circulated embedded therein. The heat transfer characteristics of the patented apparatus do not permit the high speed molding operations which can be employed in the operation of the molding apparatus of the present invention. In the operation of the patented apparatus, the metallic outer surface of the mold is gradually heated to a temperature above the chill temperature of the plastic sheet. Thus, when the mold is closed, the plastic sheet is deformed but does not take a "set" because it is not quenched to a temperature below its deformation temperature. As a result, a dwell period is required to cool the mold and plastic sheet together.